Rapid non-invasive blood pressure measuring device

ABSTRACT

A measurement device for generating an arterial volume-indicative signal includes an exciter and a detector. The exciter is adapted to receive an oscillating signal and generate a pressure wave based at least in part on the oscillating signal on the artery at a measurement site on a patient. The pressure wave includes a frequency. The detector is placed sufficiently near the measurement site to detect a volumetric signal indicative of arterial volume of the patient.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/617,648, filed Nov. 12, 2009, now U.S. Pat. No. 7,951,086, which is acontinuation of U.S. application Ser. No. 11/413,718, filed Apr. 28,2006, now U.S. Pat. No. 7,618,375, which is a continuation of U.S.application Ser. No. 11/220,035, filed Sep. 6, 2005, now U.S. Pat. No.7,041,060, which is a continuation of U.S. application Ser. No.10/685,068, filed Oct. 14, 2003, now U.S. Pat. No. 6,939,305, which is acontinuation of U.S. application Ser. No. 09/412,295, filed Oct. 5,1999, now U.S. Pat. No. 6,632,181, which is a continuation of U.S.application Ser. No. 08/672,218, filed Jun. 26, 1996, now U.S. Pat. No.6,027,452, all incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for thenon-invasive determination of blood pressure.

Direct measurement of blood pressure with a pressure measuring devicesuch as a tonometer is difficult in a clinical setting. A problem withtonometer readings is that although the times of the systolic anddiastolic pressures are correct, the pressure readings may have anincorrect scaling or have an offset in the recorded pressure. Tonometermeasurements can depend on the position of the tonometer, artery andbone structure behind the artery.

Another prior art system used to determine arterial pressures is anautomated oscillometric device called a “Dinamap” (device for indirectnon-invasive mean arterial pressure). This device is described in apaper entitled “Arterial Pressure Monitoring: Automated OscillometricDevices”; M. Ramsey III; Journal of Clinical Monitoring; Volume 7, No.1; January 1991; pp. 56-67. This system uses a cuff to supply anexternal pressure to an artery. The cuff pressure is stepped inincrements from a pressure believed to be above the systolic pressure toa pressure believed to be below the diastolic pressure. An arterialvolumetric indication is monitored by the system. For example, apressure transducer attached to the cuff will give some indication ofthe volume of the artery, since the pressure in the cuff will be greaterwhen the artery volume is high. When the mean value of the arterialblood pressure is about the same as the external cuff pressure, theamplitude of the variations of the volumetric indication will be thegreatest. In this way, an indication of the mean arterial pressure canbe obtained. A disadvantage of this prior art system is the considerabletime it takes to obtain the arterial pressure information. Many cardiaccycles are needed to obtain the data required to determine a bloodpressure.

An alternative system is described in “Vibration Technique for IndirectMeasurement of Diastolic Arterial Pressure in Human Fingers”; Shimazu,et al.; Medical and Biological Engineering in Computing; March 1999;Volume 27; pp. 130-136. This paper describes a method for obtaining adiastolic pressure which is somewhat similar to the oscillometrictechnique used with the Dinamap. In the Shimazu, et al. system, a smalloscillation is placed on the cuff pressure. A plethysmograph is used toget a volumetric indication of the volume of the artery. The output ofthe plethysmograph will show a high-frequency component imposed on apulsatile component. The cuff pressure is ramped or stepped in a mannersimilar to the Dinamap system. In the cardiac cycle where the cuffpressure is roughly equal to the diastolic pressure, the amplitude ofthe high-frequency component of the volumetric indication will begreater in the diastolic period of that cycle than at the diastolicperiod of any other cycle. In this way, the diastolic pressure can bedetermined. Like the Dinamap system, the Shimazu, et al. system isrelatively slow. Many cardiac cycles are required to determine a singleblood pressure value.

Another prior art system is described in Penàz U.S. Pat. No. 4,869,261.Penàz describes a vascular unloading system. Vascular unloading systemsattempt to cause the external applied pressure to be equal to thearterial blood pressure at all times. These systems use a plethysmographand a feedback loop in order to adjust the external pressure so that ittracks the arterial pressure. A disadvantage of this system is that,when the external pressure tracks the arterial pressure, the meanapplied pressure is relatively high. For this reason, the systemdescribed in Penàz may be uncomfortable or painful to use. Additionally,vascular unloading systems tend to produce a pressure signal that is offfrom the real arterial pressure by a DC offset.

The systems of Palti U.S. Pat. No. 4,660,544 and Sramek U.S. Pat. No.4,343,314 use very fast ramped external pressures. A disadvantage ofthese systems is that the required very fast ramped pressures may beimpractical to produce. In particular, it may be difficult to use a cuffto apply the external pressures because of the relatively long periodsof time required to inflate or deflate a cuff. Additionally, the quickexternal pressure ramp could be uncomfortable.

Therefore, it is desired to have a method and apparatus for obtaining ablood pressure that can avoid long measurement times or high appliedexternal pressures.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for quickly determininga blood pressure value at a specific time. An external pressure issupplied to the artery so that the artery experiences a range oftransmural pressures. The external pressure is set so that a knownevent, or marker, will occur during a measurement period. Themeasurement period is a short period of time that typically can be acardiac cycle or a few cardiac cycles. The value of the externalpressure at the time of the event allows for the calculation of anarterial pressure associated with the time of the event or an earlier orlater time in another cardiac cycle.

By allowing a range of transmural pressures within a cardiac cycle, thetransmural pressure is not clamped at zero like in the apparatus of thePenàz patent. In one embodiment, the range in the transmural pressure ismainly due to arterial pressure variations.

Examples of events, or markers, that can be detected include a peak inthe arterial compliance curve at the transmural pressure about equal tozero or an opening or closing of an artery. Other markers that can beused include a pressure dependant change in attenuation of a propagatingpressure wave; frequency dependent effects (i.e. level of attenuation ofa propagating wave versus frequency or a compliance versus frequencyrelationship); changes in a compliance versus attenuation relationship;a change in the viscoelastic properties of the artery or other portionof the body; or a change in the flow of blood through the artery.

Several of the preferred embodiments of the invention use, as a marker,the peak in the compliance of the artery. The peak is, by definition,the transmural pressure (or pressure across the artery wall) where theslope of the pressure-volume relationship of the artery is the steepest.The pressure-volume relationship relates the pressure across the arterywall to the volume in the lumen of the artery. The peak in thecompliance indicates that, at that transmural pressure, a given changein transmural pressure will cause a larger change in the volume of theartery than at any other transmural pressure. This peak is thought tooccur at a transmural pressure of zero because the artery wall is at theleast amount of stress at this pressure.

It is possible that the peak in compliance does not occur at zerotransmural pressure, but rather at some transmural pressure close tozero. These slight variations could be caused by a number of factorsincluding: arterial size, presence of disease states, age, gender, etc.The system of the present invention can produce relatively accurateblood pressure values when it assumes the compliance peak occurs at thetransmural pressure of zero. If significant variations in the transmuralpressure of peak compliance are found to occur, this invention wouldstill function adequately if an appropriate compensating factor wereadded to the measured pressure.

The time that an event (i.e. transmural pressure approximately zero)occurs is used to produce an indication of the arterial blood pressureat a specific time. For example, if the transmural pressure is equal tozero at time T₁, the system knows that the arterial pressure is equal tothe cuff pressure at time T₁ and/or at a different time with the sameposition in a subsequent or prior cardiac cycle as T₁.

Within as little as single cardiac cycle, all of the data required todetermine a blood pressure at a certain time can be obtained. This ismuch quicker than the many cycles required in the Dinamap and Shimazusystems.

The information of a blood pressure at a certain time can be used tocalibrate a blood pressure signal, such as a signal from an arterialtonometer, a pressure wave velocity measurement system, or a vascularunloading device. Alternatively, it may be useful just to have any bloodpressure in some situations, especially since the present invention candetermine a blood pressure so quickly.

In the present invention, it is not necessary for large changes to bemade to the external applied pressure. For this reason, the control ofthe external pressure is made easier. For example, the external pressurecan be applied by a cuff which is inflated with a fluid, such as air orwater, to a constant volume. The external pressure would thus besubstantially constant.

Some embodiments of the present invention use the fact that there is apeak to the compliance versus pressure curve at a transmural pressureapproximately equal to zero. This means that at a transmural pressureapproximately equal to zero, small arterial pressure changes can causelarge arterial volume changes. These embodiments preferably use anexternal pressure between the systolic and diastolic pressure. Thisexternal pressure can be less than the peak external pressures used inthe Dinamap or Penàz systems.

One set of embodiments using this relationship uses an arterial pressuredependent signal and an arterial volume dependent signal to produce acurve of a volume indication versus a pressure indication. Theseembodiments are called pressure/volume embodiments. The maximum rate ofchange of this curve will be at a transmural pressure approximatelyequal to zero. Alternately, the curve can be fitted to a polynomialequation. A derivative of the polynomial equation can be obtained to getan equation that is related to the compliance. A maximum of thiscompliance equation would be at a transmural pressure approximatelyequal to zero. This means that, when the arterial pressure dependentsignal has a value such that the compliance related value has a maximum,the arterial pressure is approximately equal to the externally-appliedpressure. The time at which the transmural pressure is approximatelyequal to zero is obtainable, since the system knows the value of thepressure-dependent signal at different times.

These embodiments can also work if there is a nonlinear dependence ofthe pressure-dependent signal or volume-dependent signal to the actualarterial pressure or arterial volume. This is because, in the presentembodiment, only a maxima or maximum rate of change of a curve isrequired to be determined.

Examples of arterial pressure-dependent signals that can be used includea tonometer or other pressure sensors, or a system which measures thevelocity of wave propagation in the artery. The pressure wave velocityis monotonically-related to the arterial pressure. Alternately, apressure signal from a vascular unloading device could be used as apressure dependent signal.

The volume-dependent signal can be obtained from a plethysmograph.Alternatively, the volume-dependent signal can be obtained from apressure transducer connected to the cuff used to provide the externalpressure. Increases in the arterial volume will cause the pressurewithin the cuff to increase. Decreases in arterial volume will cause thepressure in the cuff to decrease.

Another set of embodiments of the present invention use an exciter toput a high frequency pressure signal onto the blood pressure or theexternal pressure. This set of systems will then look at avolume-related signal to obtain an indication of the time that thetransmural pressure is equal to zero. The input signal is at a higherfrequency than the pulsatile components. The pulsatile componentsgenerally have relatively low frequencies. The volume-related signal canbe filtered so that the high frequency output of the volume-relatedsignal is obtained. The greatest amplitude of this high frequency signalwill be at a point of the cycle in which the transmural pressure isequal to zero. The time within a cardiac cycle that the transmuralpressure is equal to zero can be obtained with a precision that dependson the frequency of the exciter signal. The volume-related signal suchas a plethysmograph can be filtered with a high-pass filter to get thehigh frequency component.

Another set of embodiments use a system identification approach. Forexample, a constant amplitude volume oscillation to the volume of theexternal pressure cuff can cause a volume change of the artery. Apressure indicative signal can be filtered to obtain the high-frequencycomponent. The minimum amplitude of this high frequency component of thepressure changes would occur at the time of maximum compliance. Otheralternative embodiments are also possible.

Yet another set of embodiments concern the arterial closing or opening.The arterial opening and closing will usually occur at a certaintransmural pressure. A high frequency pressure signal can be senttowards an artery that has an applied external pressure. When the arterycloses, the signal will be blocked and/or reflected. This means that theexternal pressure can be set such that within a cardiac cycle, a timecan be obtained that the artery opens or closes.

Another embodiment uses a critical value associated with viscosity.Pressure wave velocity measuring system can be used to produce anindication of the attenuation of the pressure signal that is related tothe viscosity. A critical value in the viscosity can be used such thatby looking at the attenuation of the pressure signal an arterialpressure can be obtained.

In another embodiment, an event can be identified in a pressure wavethat propagates through tissue other than the artery, such as bones,skin, muscle, tendon, etc. For this system, the transfer function iscalculated for the portion of the detected pressure wave signal thattravels via paths other than the artery. The marker can be obtained bythe change in the relationship between phase, amplitude and frequency,of the propagated pressure. Another embodiment of the present inventionuses the existence of harmonics to determine a certain specific time atwhich the arterial pressure has a certain value. Non-linear transferfunctions, such as the arterial-volume/transmural-pressure curve, cancause harmonics to be produced. This means that a detector using aband-pass filter set at a frequency which is a multiple of an inputexciter frequency will produce a signal that can give information abouta critical transmural pressure, such as the transmural pressure equal tozero.

Additionally, blood flow can be manipulated to produce a marker at aparticular transmural pressure. For example, the peak flow velocity in asection of artery increases if the compliance of a distal segment of theartery were decreased. The compliance of a distal section of the arterycan be changed by applying an external pressure, and would have amaximum at a transmural pressure of zero. Other changes are measured ifthe flow meter is placed immediately distal to the segment of the arterywhere the transmural pressure was manipulated. The measurements of theblood flow can be done with a Doppler flow meter.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of the apparatus of the present invention.

FIG. 2 is a flow chart illustrating the method of the present invention.

FIG. 3 is a diagram illustrating the use of the present invention in asimultaneous measurement system.

FIG. 4 is a diagram illustrating the method of the present inventionused in a sequential measurement system.

FIG. 5 is a diagram illustrating an embodiment of the present inventionusing a tonometer to produce a pressure-indicative signal and aplethysmograph to produce a volume-indicative signal.

FIGS. 6A-D are graphs illustrating the operation of the embodiment ofthe present invention shown in FIG. 5.

FIG. 7 is a flow chart illustrating the operation of the processor usedin the embodiment of FIG. 5.

FIG. 8 is a diagram illustrating an embodiment of the present inventionusing an externally-applied signal and obtaining an arterialvolume-related signal with the use of a plethysmograph.

FIG. 9 is a diagram of an apparatus of the present invention using anexciter and a pressure transducer attached to a cuff to obtain anarterial volume indicative signal.

FIG. 10 is a graph illustrating the relationship between cross-sectionalarea or volume and the transmural pressure.

FIGS. 11A-C are graphs illustrating the operation of the embodiments ofFIG. 8 or 9.

FIG. 12 is a flow chart illustrating the operation of the processor ofFIG. 8.

FIG. 13 is a diagram illustrating an embodiment of the present inventionusing a constant volume high-frequency input to the cuff.

FIG. 14 is a graph illustrating the dependence of cross-sectional areaor volume with respect to the transmural pressure.

FIGS. 15A-D are graphs illustrating the operation of the apparatus ofFIG. 13.

FIG. 16 is a flow chart illustrating the operation of a processor usedin FIG. 13.

FIG. 17 is a diagram illustrating an embodiment of the present inventionin which a velocity-dependent measurement is made with the exciter anddetector for the velocity determining operation being placed underneathan external pressure-applying cuff.

FIG. 18 is an illustration of the velocity of propagation of a pressurewave versus the transmural pressure in the artery.

FIGS. 19A-B are graphs illustrating the embodiment of FIG. 17.

FIG. 20 is a flow chart illustrating the operation of the processor ofFIG. 17.

FIG. 21 is a diagram illustrating an embodiment of the present inventionin which a pressure wave velocity is determined to obtain apressure-related signal outside of the external pressure-applying cuffand a volume-related signal is obtained by the plethysmograph inside ofthe external pressure-applying cuff.

FIGS. 22A-D illustrate the operation of the embodiment of FIG. 21.

FIG. 23 is a diagram illustrating an embodiment of the present inventionthat operates to determine the time that an artery is opened or closed.

FIGS. 24A-C are graphs illustrating the operation of the embodiment ofFIG. 23.

FIG. 25 is a diagram illustrating an alternate embodiment of the presentinvention that operates to determine the time that an artery is openedor closed.

FIGS. 26A-B are graphs illustrating the operation of the embodiment ofFIG. 25.

FIG. 27 is a diagram of an apparatus of the present invention using aplethysmograph to obtain an arterial volume-indicative signal in twodifferent cardiac cycles with two different external pressures.

FIG. 28 is a graph illustrating the relationship between volume and thetransmural pressure.

FIGS. 29A-C are graphs illustrating the operation of the embodiment ofFIG. 27.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagram of an apparatus 30 of the present invention. Thisembodiment uses an external pressure-applying device 32, which appliesan external pressure onto an artery 34 a in a part of the body 34. Alsoshown is a bone 34 b behind the artery 34 a. The externalpressure-applying device can be, for example, a cuff, such as a Duracuffavailable from Critikon. Any other device providing a controlledexternal pressure could also be used. A detector 36 can be used todetect a physiological signal. In some of the embodiments describedbelow, detector 36 will be used to obtain an arterialpressure-indicative signal or an arterial volume-indicative signal.

Optionally, an exciter 38 can be used to produce an inducedhigh-frequency perturbation on the artery that can be detected by thedetector 36. The output of the detector 36 may be filtered. The detector36 can be positioned at positions A, B or C shown in the figure.

A processor 40 is used to analyze the data obtained by the detector ordetectors. The processor can be, for example, a computer withappropriate hardware to capture analog signals, such as an IBM PC withan analog-to-digital converter. Alternatively, a dedicated processorcould be used.

FIG. 2 is a flow chart illustrating the method of the present invention.In step 42, an external pressure is applied to an artery, the externalpressure being such that the artery experiences a range of transmuralpressures within a cardiac cycle. Allowing a range of transmuralpressures within a cardiac cycle can help avoid some of the difficultiesof the system of the Penàz patent. In some embodiments, the range oftransmural pressures is produced by applying a substantially constantexternal pressure to a section of the artery.

Steps 44-46 are preferably done by the processor 40. In step 44, anevent occurring at a transmural pressure in said range of transmuralpressures is identified, the event being associated with a first time.The event can be, for example, a peak in the arterial compliance curveat the transmural pressure equal to zero. The event could also theclosing or opening of an artery. In some embodiments, the time isdetermined without requiring data from outside a measurement period.

As will be described below with respect to the specific embodiments,this step can be done in a variety of ways. For example, data from apressure-indicative signal and a volume-indicative signal can be used toproduce a curve which indicates a time at which the transmural pressureis equal to zero, or an exciter signal component can be used to indicatea time within a cardiac cycle at which the transmural pressure is zero.

In step 46, the arterial pressure is at a specific time determined. Thisspecific time can be the time of the event identified, or it can beanother time. For example, the time of the event can be measured interms of a percentage of the cardiac cycle time from a diastolicpressure. Thus, at another time outside of the measurement period, itcan be determined that at a specific time which is the same percentageof a cardiac cycle from the diastolic pressure, the arterial pressure isthe determined value.

FIGS. 3 and 4 are flow charts illustrating different ways of using anarterial pressure indication produced by the present invention tocalibrate a pressure-tracking signal. In the simultaneous measurementsystem of FIG. 3, at the same time an external pressure is applied instep 52, and the marker event measured in step 54, a pressure-trackingsignal is determined in step 56. For example, the pressure-trackingsignal can be a signal from a tonometer; a pressure wave velocitydetermination where the pressure wave velocity is linearly related tothe pressure; or the signal from a vascular unloading device. In step58, the pressure marker, that is, the indication of a blood pressure ata time, can be used to calibrate the pressure-tracking signal. In thesequential measurement system of FIG. 4, the pressure-tracking signal isrecorded in step 60, an external pressure is applied in step 62, ameasured event determined in step 64. Next a pressure-tracking signal isrecorded again in step 66, and in step 68, the pressure marker is usedto calibrate the pressure-tracking signal. Sequential measurement, asdescribed in FIG. 4, would be advantageous for systems in which theapplication of the external pressure for determining the marker eventwould prevent an accurate pressure tracking signal.

The pressure signal to be calibrated can be from an arterial tonometer,a pressure wave velocity measurement system or a vascular unloadingsystem.

FIG. 5 is a diagram of an apparatus 70 using a tonometer 72 to produce apressure-indicative signal and a plethysmograph 75 to produce anarterial volume-indicative signal. Although the tonometer 72 gives anarterial pressure indication, tonometers by themselves are typically notsufficiently accurate. The tonometer readings depend upon theirplacement with respect to the artery and the underlying bone. For thatreason, the readings of the tonometer cannot accurately be used toproduce an actual arterial pressure. A cuff engine 74 can be used toinflate a cuff to a desired pressure, as indicated by the pressuretransducer 76. The cuff pressure is inflated to the desired externalpressure.

In a preferred embodiment, the cuff 78 is inflated to produce anexternal pressure such that the desired event will occur at least onceduring each cycle. If the desired event to be identified is thetransmural pressure equal to zero, the external pressure is maintainedat a value between the systolic and diastolic pressure. Such an externalpressure will be relatively easy to estimate. Additionally, theexternally applied pressure can be modified over different cardiaccycles to find a pressure between the diastolic and the systolic.

The plethysmograph can be a photo -plethysmograph such as a device thatuses a light emitting diode (LED) or other light source to shine lightthrough an artery to a detector. A change in the artery volume changesthe transmission of the light through the artery, producing an arterialvolume indicative signal. The plethysmograph could also be an impedanceplethysmograph. An impedance plethysmograph sends an electrical signalthrough an artery so that the detected signal provides an indication ofthe arterial volume. Additionally, a piezoelectric plethysmograph,pressure or displacement transducers or strain gages can be used.

FIGS. 6A-D illustrate the operation of the apparatus 70 of FIG. 5. FIG.6A is a pressure-indicative signal from a tonometer. FIG. 6B illustratesa volume-indicative signal from a plethysmograph. These signals can beused to produce a curve of volume versus pressure shown in FIG. 6C. Theprocessor can associate a measured pressure with a measured volumeoccurring at the same time. For example, the measured pressure P_(a)occurs at the same time T_(a) as the measured volume V_(a), and themeasured pressure P_(b) occurs at the same time T_(b) as the measuredV_(b). From such measured pressure/measured volume pairs, a graph suchas the graph of FIG. 6C can be produced. In this graph, the greatestrate of change of the measured volume versus measured pressure willoccur at a pressure P_(x) which is associated with the time T_(x). It isknown that the greatest rate of change of an arterial volume versusarterial pressure occurs at the transmural pressure equal to zero. Thus,the transmural pressure would be equal to zero at time T_(x) and thearterial pressure is equal to the applied external pressure at thistime.

Alternately, the graph of FIG. 6A can be fitted to a polynomial equationand a derivative equation produced. FIG. 6D is a graph of complianceversus measured pressure obtained with the derivative equation. At themaximum of the compliance equation, at a pressure P_(x) (associated withtime T_(x)), the transmural pressure is equal to zero.

Thus, the present invention allows for an arterial pressure indicationto be produced from data obtained over as little as a single cardiaccycle. This arterial pressure indication is not necessarily the systolicand diastolic pressure. This arterial pressure indication can be used tocalibrate a pressure-indicative signal.

Looking again at FIG. 6A, assuming that the tonometer is off from thereal pressure value by a scale constant C₁, and an offset constant C₂.The equation

real arterial pressure=C ₁ measured pressure+C ₂

will apply. The tonometer can be calibrated with two determinations ofthe real pressure obtained at different cuff pressures. For example,assuming at time T_(x), the real pressure RP_(x) is equal to the cuffpressure, CP_(x), and the measured pressure is MP_(x); and at timeT_(y), the real pressure RP_(y) is equal to the cuff pressure, CP_(y),and the measured pressure is MP_(y), then

$C_{1} = {{\frac{{CP}_{x} - {CP}_{y}}{{MP}_{x} - {MP}_{y}}\mspace{14mu} {and}\mspace{14mu} C_{2}} = \frac{{{MP}_{x}{CP}_{y}} - {{MP}_{y}{CP}_{x}}}{{MP}_{x} - {MP}_{y}}}$

Alternatively, the processor could use other well-known algorithms toscale the arterial pressure indicative signal.

If the vascular unloading system is used, the method of the presentinvention can be used to provide the correct DC offset.

A system that operates in a similar manner to that described withrespect to FIGS. 5 and 6 is described below with respect to FIGS. 21 and22. The system of FIGS. 21 and 22 uses a pressure wave velocity signalto give an indication of the arterial pressure.

FIG. 7 is a flow chart illustrating the operation of the processor 80 ofFIG. 5. In step 90, the tonometer, plethysmograph and time are sampled.The time can be a time derived from a clock signal. In step 92, orderedpairs of the sampled volume and sampled pressure can be formed into agraph of measured volume versus measured pressure as shown in FIG. 6C.In step 94, the graph is fitted with a polynomial equation that givesvolume in terms of pressure. Such a polynomial fitting to a graph is awell-known data-processing technique. In step 95, a derivative of thepolynomial equation can be produced. The production of a derivativeequation from a polynomial equation is well known. The derivativeequation will give compliance indication with respect to a measuredpressure. In step 96, the maximum of the compliance is used to give themeasured pressure at which the transmural pressure is zero. This meansthat the arterial blood pressure is equal to the external appliedpressure. A time associated with the measured pressure at the maximum isattained. In the optional step 98, the time and the pressure reading isused to calibrate a tonometer output. This can be done by producing aconstant by which all the tonometer readings are multiplied.

FIG. 8 illustrates an exciter system 100 using the exciter 102 toproduce an induced high-frequency signal which is detected byplethysmograph 104. In one embodiment, the plethysmograph produces avolumetric indication which is filtered by the high-pass filter 106 toisolate the high-frequency component induced by the exciter 102. Thishigh-frequency indication is sent to the processor 80. In a preferredembodiment, a 40 Hertz oscillator 108 is used to produce an electricalsignal to control the exciter 102 so that a 40 Hertz pressure wavesignal is imposed on the blood pressure. A 25 Hertz high-pass filter 106will pass the 40 Hz frequency components from the exciter, but willfilter out many of the lower-frequency pulsatile components. The excitercan be a small speaker having an oscillating electrical input. In oneembodiment, the exciter and detector can be at the same location.

FIG. 9 is a diagram of an apparatus 110 that uses the pressuretransducer to produce the volume-indicative signal. Thisvolume-indicative signal is filtered in the high-pass filter 106 andsent to processor 80.

FIG. 10 is a graph of the cross-sectional area versus the transmuralpressure. If a constant high-frequency pressure component is applied atdifferent baseline pressures, the high-frequency output in avolume-indicative signal will be the greatest at around zero transmuralpressure, as shown in FIG. 10.

FIGS. 11A-C are graphs illustrating the operation of the apparatus ofFIGS. 8 and 9. FIG. 11A is a graph of the arterial blood pressure withan imposed high-frequency component. As with all the arterial pressuregraphs in the present invention, this graph is simplified for thepurpose of clarity. FIG. 11B is a graph illustrating a filteredhigh-frequency component of the cross-sectional area obtained with afiltered plethysmograph signal or a filtered pressure transducer signal.FIG. 11C illustrates the amplitude of the high-frequency component ofthe volume or cross-sectional area indicative signal. Note that theamplitude is greatest at the point that the blood pressure equals thecuff pressure (transmural pressure approximately zero).

Note that by changing the cuff pressure, the diastolic or systolicpressure can be found. When the cuff pressure is reduced to thediastolic pressure, peak A will merge with peak B. When the cuffpressure is increased to the systolic pressure, peak B will merge withpeak C.

Additionally, if the cuff pressure is greater than the systolic or lessthan the diastolic, the amplitude of the high-frequency component willbe smaller and more constant without the peaks associated with atransmural pressure of around zero. Thus, the system of the presentinvention can use multiple iterations to ensure that the cuff pressureis between the systolic and diastolic.

FIG. 12 is a flow chart illustrating the operation of the processor 80of FIG. 8. In step 112, the high-frequency component is induced onto ablood pressure signal with an exciter. In step 114, a signal indicativeof the cross-sectional area is produced with the plethysmograph. Thissignal can be filtered in step 116 with a high-pass filter which filtersout pulsatile frequency components. In step 118, the filtered signal canbe sampled with an Analog-to-Digital converter, and then sent into theprocessor. Alternately, the plethysmograph signal itself can be sampledin step 120, and this sampled signal filtered within the processor toremove the pulsatile components in a step 122. In step 124, a time thatthe amplitude of the filter signal is a maximum is determined. Thetransmural pressure at this time is zero. Thus, the arterial bloodpressure at this time is equal to the external pressure.

Another set of embodiments use a system identification approach.Typically, in system identification, a system is perturbed and aresponse is measured. For example, the compliance can be directly probedby perturbing the artery with either a pressure or volume perturbationand recording the resulting volume or pressure response.

One way to do this is to provide a constant amplitude high-frequencyvolume perturbation and record the resulting pressure response. Anexample of such an approach is described below with respect to FIG. 13.FIG. 13 is a diagram illustrating an apparatus 126 which uses a device128 to produce a small constant amplitude change in the cuff volume. Thevolume of the cuff vibrates a small amount due to the induced volumechange by the device 128. In a preferred embodiment, a standardoscillometric cuff 78 is modified to provide a constant volumeperturbation. This modification consists of device 128 that preferablycomprises a piezoelectric speaker (not shown) attached to the cuff suchthat the diaphragm of the speaker forms part of the outside wall of thecuff. Movement of the speaker diaphragm changes the volume of the cuff.The displacement of the cuff 78 is sensed by an accelerometer (notshown) that is used in a feedback circuit to control the displacement ofthe diaphragm and ensure that it is constant even when the pressureinside the cuff changes. The device 128 also could be a piston (notshown) attached to a rotating disk (not shown) to produce a constantvolume change in the cuff 78.

The cuff 78 is inflated by a cuff engine 74 that can inflate the cuff toa predetermined pressure and then close a valve (not shown) to seal thecuff volume. The cuff engine 74 and valve are connected to the processor80 by a serial cable. This allows the processor 80 to control theinflation of the cuff 78 and the closing of the valve.

The pressure inside the cuff 78 is sensed by a pressure transducer 76,such as Micro Switch Model No. 142PC15G, mounted inside the cuff. Aprocessor 80 that is capable of capturing analog waveforms andperforming mathematical computations is used to record and process thesignals. This processor 80 could be a general purpose computer that hasadditional analog-to-digital hardware, or it could be a customprocessor.

The device works as follows. The cuff 78 is wrapped around the patient'sarm and secured with a Velcro T™ strap. The processor 80 sends a signalto the cuff engine 74 to inflate the cuff 78 to some value between thesystolic and diastolic pressure. The desired DC cuff pressure could bedetermined by either a previous blood pressure measurement, from thecalibrated prediction of propagation velocity, tonometer or vascularunloading system, or it could be determined iteratively throughsuccessive attempts at measuring the pressure with the presentinvention. The piezoelectric speaker is activated at this point toprovide the constant amplitude volume perturbation. This perturbation istypically a sinusoidal oscillation at about 25 Hertz, but could be avariety of waveforms. The amplitude of the recorded pressure signal willdepend upon the compliance of the cuff. This compliance is largelydetermined by the compliance of the arm, since the arm is supporting oneside of the cuff. The arm compliance is partially determined by thecompliance of the artery. Thus, the pressure inside the cuff ispartially determined by the compliance of the artery. When the arterycompliance is a minimum, the pressure oscillation will be minimum. Theprocessor 80 records the 25 Hertz pressure oscillations in the cuff forat least one cardiac cycle. The processor 80 then calculates theamplitude of this pressure oscillation as a function of time. This canbe done in a variety of ways, the simplest of which is to calculate thepeak-to-peak amplitude for each cycle (this is the difference betweenthe minimum and maximum values of the pressure signal over afour-millisecond period). The time at which the pressure oscillation isa minimum indicates the time when the compliance is maximum and thus thetime when the DC cuff pressure is approximately equal to the appliedpressure.

It should be noted that this system identification approach to probingcompliance can be accomplished in a number of ways. In this embodiment,the volume was oscillated and the resulting pressure was recorded. Itis, of course, possible to perturb or oscillate the pressure, andmeasure the arterial volume oscillation response to find the peak incompliance. In addition, it is usually not necessary to control theperturbation signal, as illustrated in this embodiment. It is usuallysufficient to measure the perturbation amplitude and track the ratio ofthe output response to the input response. It also should be noted thatthe relationship between the input and output need not be described by asimple ratio of amplitudes, but the relative phases can be accounted forby more complicated mathematical descriptions such as complex transferfunctions, auto-regressive moving average (ARMA) models or othermathematical descriptions.

FIG. 14 is an illustration of the cross-sectional area versus transmuralpressure. Notice that a constant amplitude high-frequency component of across-sectional area signal input produces the smallest high-frequencycomponent pressure signal output when the baseline transmural pressureis at zero. This fact is used in the operation of the embodiment of FIG.13.

FIGS. 15A-D illustrate the operation of the embodiment of FIG. 13. FIG.15A shows a high-frequency volume signal which illustrates the change inthe volume of a cuff. FIG. 15B illustrates the blood pressure. FIG. 15Cillustrates a cuff pressure showing the imposed high-frequencycomponent. FIG. 15D is a filtered high-frequency pressure transducersignal. This figure illustrates that the amplitude of this signal islowest at the points at which the blood pressure is equal to thetransmural pressure.

FIG. 16 illustrates a method of operation of the apparatus shown in FIG.13. In step 130, high-frequency components are induced in the volume ofthe cuff used to produce the external pressure onto the artery. In step132, a signal is produced from the pressure transducer indicative of thepressure in the cuff. In step 134, this signal is filtered to take outthe constant and pulsatile components of a signal with a high-passfilter. In step 136, this filtered signal is sampled by anAnalog-to-Digital converter (not shown) and sent to the processor.Alternatively, the pressure transducer signal can be sampled in step 138and then sent to the processor. The processor can filter out theconstant and pulsatile components of the sample signal in step 140. Instep 142, the processor determines a time at which the amplitude of thefiltered signal is at a minimum. At this time, the transmural pressureis zero.

Those skilled in the art will realize that other system identificationembodiments can be used with the present invention. For example,although the above system measures a pressure response to a volumeperturbation, it is also possible to have a pressure perturbation andmeasure a volume response. Additionally the perturbation need not becontrolled or held constant. Measuring the perturbation is sufficient inmost cases.

FIGS. 17 and 21 are diagrams that illustrate embodiments which usepressure wave velocity measurement systems. In FIG. 17, an apparatus 144has a device for measuring the velocity of a high-frequency pressureoscillation along the artery comprising exciter 146 and detector 148.The system also includes a cuff 78 for applying external pressure to theartery, a cuff engine 74 to inflate the cuff 78 to a predeterminedpressure, and a processor 80 to control the cuff engine 74 and captureand process the velocity signal.

The propagation velocity measuring device measures the velocity ofpropagation of a high-frequency pressure oscillation along the artery.The pressure oscillation is induced by an exciter 146 at one point inthe artery and the propagated pressure oscillation is sensed at anotherpoint in the artery by a detector 148. The velocity of the propagationis calculated from the distance between the exciter 146 and the detector148 and the difference in phase between the excitation and detectionpoints. The velocity of the propagation is related to the arterialpressure. This device is described in much more detail in thebelow-referenced U.S. patent application Ser. No. 08/228,213 and itsCIP's Ser. No. 08/561,923 and Ser. No. 08/556,547. The velocitymeasurement systems can operate at a variety of oscillation frequenciesranging from 100 to 1000 Hertz.

In a preferred embodiment, the propagation velocity system is placed onthe forearm of the patient. The detector 148 is placed over the radialartery on the styloid process of the radial bone. The exciter 146 isplaced approximately 5 cm proximal to the detector 148, also over theradial artery. The cuff 78 is wrapped around the patient's arm to coverthe exciter 146 and detector 148 of the velocity measurement system. Thecuff 78 can alternately be positioned in between the exciter 146 anddetector 148.

The velocity of propagation of the high-frequency pressure oscillationis inversely related to the compliance of the artery, because as theartery compliance increases the velocity decreases. Since the arterialcompliance has a peak at a transmural pressure of approximately zero,the propagation velocity will have a minimum at this transmuralpressure.

The device works as follows. The processor sends a signal to the cuffengine 74 causing the cuff engine 74 to inflate to a predeterminedpressure between the systolic and diastolic pressure. This DC cuffpressure could be determined by either a previous blood pressuremeasurement, from the calibrated prediction of propagation velocity,tonometer or vascular unloading system, or it could be determinediteratively through successive attempts at measuring the pressure withthe present invention. The processor then records the signal from thepropagation velocity measurement system for at least one cardiac cycle.The processor then determines the time during the cardiac cycle that thevelocity was minimum. The time of this minimum velocity is the time whenthe applied pressure is approximately equal to the arterial pressure.Once this determination has been made, the cuff 78 can be deflated.

FIG. 18 illustrates the propagation wave velocity in the artery versusthe transmural pressure. Note that this curve has a minimum at atransmural pressure of about zero. This means that the output velocityof the propagation wave will have a minimum when the transmural pressureis equal to zero.

FIGS. 19A and 19B are graphs illustrating the operation of the apparatusof FIG. 17. As shown in these graphs, when the arterial pressure isequal to the external pressure, the velocity of propagation will have aminimum value.

FIG. 20 is a flow chart illustrating the use of the apparatus of FIG.17. In step 152, an exciter and detector 148 are used to produce anindication of the velocity of a pressure wave transferred through theartery. In step 154, an external pressure is applied which is betweenthe systolic and diastolic pressure. In step 156, a minimum of thevelocity measurement is used to obtain dtime that the transmuralpressure is equal to zero.

FIG. 21 is a diagram illustrating an apparatus 158 using an exciter 160and a detector 162 which are not placed underneath the external pressurecuff. This system is a pressure/volume embodiment. These pressure/volumeembodiments require two signals, one that is related to the arterialpressure, and one that is related to the arterial volume. The volumemeasurement should be taken in the region over which the externalpressure is applied. The external pressure should be within the systolicand the diastolic pressure. Another example of a pressure/volumeembodiment is discussed above with respect to FIGS. 5-7.

In the preferred embodiment, the cuff 78 is a standard oscillometriccuff such as that provided by Critikon for their Dinamap product. Thecuff engine 74 is capable of inflating the cuff to a pressure up to 300mmHg, and the pressure transducer 76 should have a range up to 300 mmHgand sufficient sensitivity to measure pressure changes as small as 0.1mmHg. The processor 80 should be able to capture analog waveforms andperform mathematical computations.

The cuff engine 74 is preferably under the control of the processor 80and has a valve (not shown) that can seal off the cuff 78 so that thevolume of the cuff 78 can be fixed. This valve (not shown) should beunder control of the processor 80.

The processor 80 can be a general-purpose computer, such as an IBM PC,with a multi-function input-output board, such as a National InstrumentsMIO-16 board. Alternately, a custom microprocessor could be used.Communication to the cuff engine can be by a serial connection betweenthe processor and the cuff engine.

The propagation velocity measuring device measures the velocity ofpropagation of a high-frequency pressure oscillation along the artery.The pressure oscillation is induced by an exciter 160 at one part of theartery and the propagated pressure oscillation is sensed in anotherpoint along the artery by detector 162. The velocity of propagation iscalculated from the distance between the exciter 160 and detector 162and the difference in phase between the excitation and detection points.

Pressure wave velocity measurement systems are described in more detailin U.S. patent application Ser. No. 08/228,213 entitled “Apparatus andMethod for Measuring an Induced Perturbation to Determine BloodPressure” filed Apr. 15, 1994 (corresponding to InternationalPublication WO95/28126), and its CIP's Ser. No. 08/561,923 entitled“Apparatus and Method for Measuring an Induced Perturbation to Determinea Physiological Parameter” and Ser. No. 08/556,547 entitled “Apparatusand Method for Measuring an Induced Perturbation to Determine aPhysiological Parameter,” which are incorporated herein by reference.These systems sometimes have amplitude indications which are availableto obtain viscosity information as well.

In a preferred embodiment, the velocity measurement system could beplaced over the radial artery on the forearm. The cuff can be wrappedaround the wrist, just distal to the exciter.

This embodiment works as follows. The processor sends a signal to thecuff engine to inflate the cuff to some value between the systolic anddiastolic pressure. This DC cuff pressure could be determined by aprevious blood pressure measurement, from the calibrated prediction ofpropagation velocity, tonometer or vascular unloading system, or it canbe determined iteratively through successive attempts at measuring thepressure with the present invention. Once the pressure in the cuff hasbeen established, the cuff engine closes the valve that seals off thecuff, fixing the volume of air in the cuff.

With the cuff volume fixed, the pressure in the cuff will have a smallpulsatile component that is related to the volume of the artery. The DCcomponent of the cuff pressure can be removed by the processor, leavingthe pulsatile component. This pulsatile component is related to thevolume of the artery.

The processor simultaneously records the pulsatile portion of the cuffpressure and the propagation velocity. These two signals correspond tothe volume-indicative signal and the pressure-indicative signal. Theprocessor then determines the slope of the relationship between volume-and pressure-indicative signals by taking the difference betweensuccessive pairs of measurements. This slope is related to thecompliance and is defined as

${C(k)} = \frac{{V(k)} - {V( {k - 1} )}}{{P(k)} - {P( {k - 1} )}}$

where V(k) is the volume-indicative signal at sample k, and P(k) is thepressure-indicative signal at sample k.

The time at which this quantity peaks is the time at which the appliedpressure is approximately equal to the arterial pressure. Once the peakin compliance is found, the cuff pressure can be set to zero.

FIGS. 22A-D are graphs which illustrate the workings of the embodimentof FIG. 21. In FIG. 22B, the graph of the measured velocity is shown.Looking again at FIG. 21, the pressure wave velocity is measured in aregion with no external pressure applied. This means that, as shown inFIG. 18, the transmural pressure will be equal to the arterial pressure,which ranges, for example, from about 80 mmHg to 120 mmHg. Thus themeasured velocity will be proportional to the arterial pressure. Themeasured volume shown in FIG. 22C is measured in a pressure regime wherean external pressure is applied by a cuff and where the externalpressure is between the systolic and diastolic pressure. This means thatthe volume signal of FIG. 22C has at least one point where thetransmural pressure is equal to zero. In a similar fashion to thatdescribed with respect to FIGS. 6A-D, a graph of the measured volumeversus measured velocity can be produced. This graph will have thegreatest rate of change at a measured velocity Vel_(x), associated witha time T_(x), at which the transmural pressure under the cuff is equalto zero or the arterial pressure everywhere is equal to the externalapplied cuff pressure. The known arterial pressure at measured velocityVel_(x) can be used to calibrate the velocity/arterial-pressure curve.

FIG. 23 is a diagram showing apparatus 158 that illustrates anembodiment based upon the arterial closing pressures. In the embodimentshown in FIG. 23, an exciter 60 under the control of an oscillator 162produces an excitation signal which is passed down the artery anddetected by detector 164. The output of detector 164 is filtered, in themanner described in “Apparatus and Method for Measuring an InducedPerturbation to Determine Blood Pressure” International Publication WO95/28126, to determine the portion of the high-frequency signal thatpasses through the artery. This filter removes the detectedhigh-frequency components that pass through tissue other than theartery. Processor 80 uses the output of detector 164 to determinewhether the artery is closed.

FIGS. 24A-C illustrate the operation of the embodiment shown in FIG. 23.FIG. 24A illustrates the arterial pressure. FIG. 24B illustrates thehigh-frequency components of the excitation onto the artery that iscaused by the exciter 160. FIG. 24C illustrates the high-frequencycomponent of the pressure wave passing through the artery to detector164. Note that, at an artery closing pressure, the signal is reduced tozero or severely attenuated. The processor 80 can then determine that,at time T₁, the transmural pressure is at the transmural pressure thatcauses the artery to close. Thus, the arterial pressure at time T₁ isthe cuff pressure plus the artery closing transmural pressure. In thismanner, the arterial pressure at a given time can be determined.

The pressure can be applied in a variety of locations: over the detectoror exciter; in the propagation region; just before the exciter or justafter the detector. The different placements will show differentresponses in the propagated signal (positioning the pressure at thedetector, in the propagation region or at the exciter will showattenuation, while positioning the pressure before the exciter or afterthe detector will show amplification because of reflections), but allcould potentially be used to detect opening and/or closing.

FIG. 25 is a diagram illustrating an alternate embodiment of the presentinvention that operates to determine the time that an artery is openedor closed. In the embodiment shown in FIG. 25, a detector 166 is used todetect the arterial pulse signal.

FIGS. 26A-B are graphs illustrating the operation of the embodiment ofFIG. 25. FIG. 26A illustrates the arterial pressure. FIG. 26Billustrates the pulse signal detected at detector 166. At an arteryclosing pressure, the signal is reduced to zero or severely attenuated.The processor 80 determines, at time T₁, the transmural pressure is atthe artery closing transmural pressure. In this manner, the arterialpressure at a given time can be determined. The pressure can be appliedover the detector or before the detector.

FIG. 27 is a diagram of an apparatus 170 of the present invention usinga plethysmograph to obtain an arterial volume indicative signal in twodifferent cardiac cycles with two different external pressures. One ofthe external pressures is less than the diastolic pressure and the othertransmural pressure is between the diastolic and the systolic pressure.

FIG. 28 is a graph illustrating the relationship between arterial volumeand the transmural pressure.

FIGS. 29A-C are graphs illustrating the operation of the embodiment ofFIG. 27. FIG. 29A illustrates the arterial pressure. At time T_(o), theexternal pressure is set at 70 mmHg, a pressure below the diastolic. Attime T_(L), the external pressure is set at 90 mmHg, a pressure betweenthe diastolic and the systolic pressure. FIG. 29B showsvolume-indicative signals from the plethysmograph. The top curve is thearterial volume-indicative measurement at a cuff pressure of 70 mmHgstarting at time T_(o). The bottom curve is the arterialvolume-indicative measurement at a cuff pressure of 90 mmHg starting attime T_(L). The processor 80 stores the arterial volume-indicativemeasurements to compare measurements made at different cycles. Note thatthe arterial volume measurements are relatively linearly related fromtime +T₁ to time +T₂, this range corresponding to the arterial pressuresabove 90 mmHg. When the arterial pressures go below 90 mmHg, thevolume-indicative output of the measurements starting at time T_(L)drops sharply. As shown in FIG. 29C, the volume indications at differentexternal pressures can be graphed against each other by the processor80. A linear region of this graph corresponds to the region between time+T₁ and time +T₂. A nonlinear region of this graph corresponds to theregion between time +T₂ and time +T₃. The time that the arterialpressure equals the external pressure (90 mmHg) can be determined fromthis graph by processor 80. There is a change in the slope at the pointthat the transmural pressure is equal to zero. Note that it is alsopossible to use such a system with two plethysmographs, one having anexternal applied pressure and the other not.

Various details of the implementation and method are merely illustrativeof the invention. It will be understood that various changes in suchdetails may be within the scope of the invention, which is to be limitedonly by the appended claims. For instance, as discussed above, thepresent invention could use a relationship between the viscosity and acritical pressure (such as a transmural pressure equal to zero) todetermine a blood pressure at a certain time. Or the detector could beplaced in a location which would allow it to receive pressures whichpass through not only the artery, but skin, muscle, bone, or othertissue. Alternately, for the induced signal cases, a non-linear transferfunction, such as the arterial volume versus transmural pressure, can beused to produce harmonics which can be detected using a band-pass filterset at a multiple of the input frequency, to give a signal that couldgive information about a critical transmural pressure.

Additionally, measurement of the actual flow of the blood can be donewith a Doppler flow detector. The blood flow is related to the arterialvolume and pressure in such a manner that there is a critical value thatcan be detected as an event.

1. A measurement device for determining blood pressure, the measurementdevice comprising: an external pressure device configured to apply afirst external pressure to an artery and subsequently apply a secondexternal pressure to the artery, the first external pressure and thesecond external pressure being such that the artery experiences a rangeof transmural pressures; and a processor, wherein the processor isconfigured to identify a first time when transmural pressure isapproximately equal to zero and a second time when transmural pressureis approximately equal to zero.
 2. The measurement device of claim 1,wherein the external pressure device comprises a cuff.
 3. Themeasurement device of claim 1, wherein the applied second externalpressure is greater than the applied first external pressure.
 4. Themeasurement device of claim 1, wherein the applied first and secondexternal pressures are between a systolic and diastolic arterialpressure.
 5. The measurement device of claim 1, wherein the processor isconfigured to determine a first arterial pressure at the first time anda second arterial pressure at the second time.
 6. The measurement deviceof claim 5, wherein the processor is configured calibrate apressure-indicative signal using the first and second arterialpressures.
 7. The measurement device of claim 5, further comprising adetector to produce a pressure-indicative signal, wherein the processoris further adapted to determine a user's blood pressure based on thepressure-indicative signal and the first and second arterial pressures.8. The measurement device of claim 7, wherein the detector comprises atonometer.
 9. The measurement device of claim 7, wherein the detectorcomprises a pressure wave velocity measurement system.
 10. Themeasurement device of claim 5, wherein the processor is configured todetermine a patient's blood pressure based on a pressure-indicativesignal received from a detector and the first and second arterialpressures.
 11. The measurement device of claim 5, wherein the processoris configured to determine an offset of a pressure-indicative signal.12. The measurement device of claim 5, wherein the processor isconfigured to scale a pressure-indicative signal.
 13. The measurementdevice of claim 1, wherein the processor is adapted to identify a changein the viscoelastic properties of an artery.
 14. The measurement deviceof claim 1, further comprising a first detector electrically connectedto the processor to produce a signal indicative of arterial pressure anda second detector electrically connected to the processor to produce asignal indicative of an arterial volume.
 15. The measurement device ofclaim 1, further comprising an exciter configured to perturb at leastone of pressure and volume of a patient's artery with an oscillatingperturbation signal.
 16. A method of determining arterial pressure,comprising: applying a first external pressure to an artery, the firstexternal pressure being such that the artery experiences a range oftransmural pressures; identifying a first time when transmural pressureis approximately equal to zero; applying a second external pressure tothe artery, the second external pressure being such that the arteryexperiences a range of transmural pressures; and identifying a secondtime when transmural pressure is approximately equal to zero.
 17. Themethod of claim 16, wherein the second external pressure is greater thanthe first external pressure.
 18. The method of claim 16, wherein thefirst and second external pressures are between a systolic and diastolicarterial pressure.
 19. The method of claim 16, further comprisingdetermining a first arterial pressure at the first time and a secondarterial pressure at the second time.
 20. The method of claim 19,further comprising determining a patient's blood pressure based on apressure-indicative signal received from a detector and the first andsecond arterial pressures.